1. Field of the Invention
The present invention relates to a backlight unit for a liquid crystal display (LCD) using a light emitting diode (LED) and a light guide plate, more particularly, which can obtain high definition through a high contrast ratio and be reduced in thickness.
2. Description of the Related Art
Recently, with the trend of a thinner and high-performing image display device, a liquid crystal display is prominently utilized in televisions, monitors and the like. A liquid crystal panel does not generate light on its own so that the liquid crystal display requires a backlight unit (BLU). The BLU has adopted as its light source a cold cathode fluorescent lamp (CCFL) which is inexpensive and easy to assemble together. The BLU using the CCFL performs separate driving such as local dimming or impulsive driving. Besides, the CCFL-based BLU is not environment-friendly due to use of a mercury gas, and low in response rate. To overcome this drawback, the CCFL has been replaced by light emitting diodes (LEDs) as the light source for the BLU.
Meanwhile, in a method to drive the BLU by local dimming, the liquid crystal panel of the liquid crystal display is divided into a plurality of areas and brightness of the BLU light source can be adjusted according to the divided areas by a value of a grey level. That is, light emitting diodes (LEDs) of the BLU corresponding to a brighter area in the displayed image are turned on, and the LEDs corresponding to the rest of the image are turned on with low brightness or completely turned off. In the impulsive driving, the BLU is time-synchronized with the liquid crystal panel. Here, a plurality of light source areas arrayed in rows and columns on a BLU board are turned on sequentially.
In general, the BLU includes a direct-type BLU and an edge-type BLU. In the latter, a bar-shaped light source is disposed at an edge of the liquid crystal panel to irradiate light thereonto through a light guide plate. On the other hand, in the former, a surface light source is disposed below the liquid crystal panel to directly irradiate light thereonto.
FIG. 1 is a perspective view illustrating a conventional edge-type BLU using an LED. Referring to FIG. 1, the BLU 10 includes a light guide plate 11, LED light sources 15 and 17 disposed along edges of the light guide plate 11, respectively, and a reflective plate 19 disposed underneath the light guide plate 11. Each of the LED light sources 15 and 17 includes a printed circuit board 17 and a plurality of LEDs 15 arranged on the board 17. Light incident on the light guide plate 11 from the LEDs 15 is internally total-reflected and scattered, and then transferred to the liquid crystal panel. This edge-type BLU 10 can be manufactured in a relatively small thickness but not suitable for separate driving such as local dimming.
FIG. 2 is a perspective view illustrating a conventional direct-type BLU using an LED. Referring to FIG. 2, the BLU 20 includes a PCB 21 and a plurality of light emitting diodes 23 arranged thereon. A diffusing plate 25 is disposed between a liquid crystal panel (not illustrated) and the LEDs to diffuse light. The LEDs 23 directly irradiate light onto the liquid crystal panel. This direct-type BLU 20 can perform separate driving such as local dimming. In order to achieve such local dimming, each of the LEDs 23 can be controlled to be turned on/off. Alternatively, the BLU is divided into predetermined areas, e.g., A1, A2, and A3 areas to drive the LEDs 23 according to the divided areas. However, the LEDs 23, when driven respectively, entail high consumption power, costs for a heat radiating structure for coping with high temperature and complicated circuits. Meanwhile, the LEDs, when driven according to the divided areas, experience less local dimming effects due to difficulty in dividing the areas and thickness (H) of the BLU. Notably, here, the BLU should be sufficiently thick to assure uniform light, thereby disadvantageous for thinning the BLU and the liquid crystal display.